Airbags for motor vehicles are known and have been used for a substantial period of time. These devices are installed on the driver and passenger side of automobiles and, in the event of a collision, are rapidly inflated with gas, to act as an energy absorbing barrier between the driver or passenger and the steering wheel or dashboard of the automobile.
With the advent of new curtain-type airbags comprising a plurality of pillows formed from stitched areas of the bag article (to protect passengers during roll-over and side impact collision events), there is now a greater emphasis on providing such curtain-type airbags which will retain their inflation pressure for extended period after deployment, and will perform if and when necessary even upon storage of a long duration (years for example). Such pillowed fabrics thus comprise seams which control the shape and size of the inflated cushion. Upon inflation of such specific airbag cushions, pressure may be applied in great force, particularly on the seams, during an inflation in response to a collision event. These stitched areas thus must retain their strength upon inflation, thereby setting forth the requirement that the individual yams at such seams will not become unraveled (i.e., "combed-out") easily (which would result in the leakage air at too great a rate to afford sufficient protection), particularly upon inflation of the airbag. With such an expansion in stitching requirements as compared with traditional driver-side and passenger-side airbags, the areas for potential air leakage have also increased dramatically.
In the past, coatings have been applied to fabrics, intended for use in automotive airbags, to resist the unwanted permeation of air through the fabric and, to a lesser extent, to protect the fabric from detriment by the hot gases used to inflate the bags. Polychloroprene was the polymer of choice in the early development of this product, but the desire to decrease the folded size of the completed airbag, and the tendency of polychloroprene to degrade, with exposure to heat, to release the components of hydrochloric acid (thereby potentially degrading the fabric component as well as releasing hazardous chemicals), has led to the almost universal acceptance of silicone (polydimethylsiloxane or similar materials) as a more suitable coating. In the quest for the most compact folded size possible, coating levels of polymer have dropped from around 2.5 ounces per square yard of fabric, to levels approaching 0.5 ounces per square yard.
New developments in airbag technology, particularly newer designs being placed in the sides of the passenger compartment (as noted above), have introduced the requirement that the bags hold pressure longer under use. This, and the evolution of the lower coating levels of silicone polymer, have begun to highlight the effect that, when a sewn seam is put under stress, a naturally lubricating silicone coating may allow the yams from which the fabric is constructed to shift. This shifting can lead to leakage of the inflating gas through the new pores formed from the shifting yarns, or, in drastic cases, cause the seam to fail. Since the airbag must retain its integrity during a collision event, in order to sufficiently protect the driver or passenger, there is a great need to provide coatings which provide both effective permeability characteristics and sufficient restriction of yam shifting for the airbag to function properly, if and when necessary. In recent years, silicone coatings have been utilized to provide such desired permeability and strength characteristics. However, the relative cost of such coating materials (such as polydimethylsiloxane) is sufficiently high that new, more inexpensive alternatives are being sought. Thus, there exists a need for providing good adhesion and a strong bond between the individual yarns (in order to effectuate long-term rigidity of the fibers to prevent unraveling) at cut edges or at seams while simultaneously providing aging stability and excellent low air permeability characteristics. Such a necessary improvement has not been afforded the airbag industry within the prior art. However, the inventive two-layer coating system does provide the necessary strength, durability, permeability, and reliability to the airbag industry, particularly for large-scale production of heavily stitched curtain-type airbags.